Definition of tetragon :
1. A plane figure having four sides and angles; a quadrangle, as a square, a rhombus, etc.
2. An aspect of two planets with regard to the earth when they are distant from each other ninety degrees, or the fourth of a circle.
geometric figure (part of speech: noun)
dodecahedron, parallelogram, parallelepiped, pentahedron, octagon, right-angle triangle, hexahedron, polygon, pentagon, quadrilateral, square, hexagon, trapezoid, rhombus, cube, quadrangle, equilateral triangle, triangle, rectangle, heptagon, dodecagon, cusp, scalene triangle, tetragram, polyhedron, tetrahedron, isosceles triangle, decagon, octahedron